1. Field of the Invention
The present invention relates to a control method for an injection motor of an injection-molding machine, and more particularly, to a method for controlling an injection motor having an overheat preventing function, whereby injection-molding conditions are set securely such that the injection motor is prevented from overheating.
2. Description of the Related Art
In an injection-molding machine, as is generally known, a synchronous motor (hereinafter referred to as an injection motor) is used as a drive source for an injection unit. Processes of injection, hold, and application of back pressure for metering are executed by means of the injection motor. In the injection-molding machine of this type, the output torque of the injection motor is small, so that a very small driving current flows through the injection motor, during the injection process or when the back pressure is applied. In the hold process, a great output torque must be produced, so that a very great driving current flows. Meanwhile, the injection motor is selected in consideration of various design conditions, such as economical efficiency. It is uncommon, therefore, to use, as the injection motor, a large-capacity motor such that its continuous rated current is never exceeded by the driving current. As a result, the driving current during the hold process may possibly exceed the continuous rated current by a large margin, so that the average value of the driving current flowing through the injection motor during one injection-molding cycle may sometimes exceed the continuous rated current. In such a case, the injection motor may suffer from overheating or some other problems.